The transport properties of cell membranes are of great importance since it is these properties which maintain a suitable internal environment for the metabolic activities of all cells. For those molecules of greatest metabolic importance, such as sugars and amino acids, specific transport systems are often present in the membrane. However, for many low molecular weight species, such as drugs and pesticides, specific transport systems most likely do not exist. The entry rate of such molecules into cells is most likely governed by passive diffusional transport through cell membranes. The object of the proposed research is to investigate the passive diffusional transport properties of synthetic and natural membranes. Phospholipids with covalently attached fluorescent probes will be synthesized. A wide variety of molecules function as collisional quenchers of fluorescence. Fluorescence lifetime measurements will yield the probe-quencher collisional rate. Independent measurements of the quencher's (permeant) lipid-water partition coefficient will define its membrane diffusion coefficient. Using phospholipid vesicles of varying composition and natural membranes we will investigate the size, shape. polarity and functional group dependence of passive diffusion within membranes. By location of the fluorescence probe in the hydrophobic or polar region of the phospholipid we may be able to detect the localization or specific associations of drug-like or pesticide-like quenchers with membrane components. For amphipathic molecules we will attempt to distinguish lateral diffusion on one side of the bilayer from transport across the bilayer. Additionally, we will investigate the ability of serum lipoproteins and albumins to exchange foreign hydrophobic molecules into membranes, and thus serve as a transport mechanism.